Air/fuel ratio control device for internal combustion engine

ABSTRACT

An air-fuel ratio control apparatus for an internal combustion engine, implementing integral correction of the air-fuel ratio by an integral term edfii obtained by multiplying an integrated difference between a target air fuel ratio and the actual air-fuel ratio by an integral gain, wherein the upper and lower limit values of the integral term are set based on the actual intake air amount and the actual air-fuel ratio. This limits the range of the integral term edfii to prevent it from being set at an excessively high or low level removed from the realities of the intake air amount and the air-fuel ratio, and thereby to prevent erroneous air-fuel ratio correction by the integral term.

FIELD OF THE INVENTION

The present invention relates to an air-fuel ratio control apparatus foran internal combustion engine, implementing integral correction ofair-fuel ratio by an integral term obtained by multiplying an integrateddifference between a target and actual air-fuel ratios by an integralgain.

BACKGROUND OF THE INVENTION

As is well known, some internal combustion engines for vehicles or thelike clean up exhaust gas using a three-way catalyst that simultaneouslyenhances oxidation of unburned components (HC and CO) and reduction ofnitrogen oxides (NOx). In order to maintain the purification performanceof such a three-way catalyst, it is necessary to combust fuel at anair-fuel ratio that is close to the stoichiometric air-fuel ratio.Therefore, an internal combustion engine equipped with a three-waycatalyst performs feedback control such that the air-fuel ratio seeksthe stoichiometric air-fuel ratio, while detecting an air-fuel ratioobtained based on oxygen concentration of exhaust gas.

Recently, a three-way catalyst provided with oxygen storage capacity hasbeen commercialized. Such a three-way catalyst stores excessive oxygenwhen the air-fuel ratio is leaner than the stoichiometric air-fuel ratioand the oxygen concentration in exhaust gas is high, and releases thestored oxygen to compensate for shortage of oxygen when the air-fuelratio is richer than the stoichiometric air-fuel ratio and the oxygenconcentration is low. This suitably maintains exhaust gas purificationcapacity for the catalyst even when the air-fuel ratio temporarilydeviates from the stoichiometric air-fuel ratio. However, because oflimited oxygen storage capacity of the catalyst, it is necessary to keepthe quantity of oxygen stored by the catalyst in a certain range (e.g.,about half of its maximum capacity) to ensure that the catalyst canstore or release oxygen on a steady basis.

Therefore, control apparatuses that perform air-fuel ratio feedback byPI control or PID control have been proposed for internal combustionengines, as disclosed by, e.g., Japanese Laid-Open Patent PublicationNo. 9-280038. Such a control apparatus controls air-fuel ratio by anintegral action on a difference detected between a target and actualair-fuel ratios. A PI control system, for example, corrects an air-fuelratio based on a correction amount obtained using the following formula(1):Air-fuel ratio correction amount=(Air-fuel ratiodifference)×(Proportional gain)+(Integrated air-fuel ratiodifference)×(Integral gain)  (1)

In the formula (1), the first term of the right-hand side [(Air-fuelratio difference)×(Proportional gain)] is a proportional term, based onwhich deviation of air-fuel ratio from the stoichiometric air-fuel ratiois compensated. The second term [(Integrated air-fuel ratiodifference)×(Integral gain)] is an integral term, based on which steadystate deviation of the air-fuel ratio is compensated. More specifically,the integral term corrects air-fuel ratio in such a way as to equalizean integrated quantity of oxygen newly stored by a three-way catalystwith an integrated quantity of oxygen released from the catalyst.Therefore, integral correction of air-fuel ratio stably maintains thequantity of oxygen stored by a three-way catalyst.

It should be noted, however, that an integral term for integralcorrection of air-fuel ratio is determined based on the history ofair-fuel ratios irrespective of the actual intake air amount or air-fuelratio, which may lead to erroneous air-fuel ratio correction, asdescribed below.

When an internal combustion engine whose air-fuel ratio tends to greatlydeviate from the stoichiometric air-fuel ratio is operating at a highintake air amount, this may cause a relatively large absolute value ofthe integral term. When the engine is decelerated in this state, and theintake air amount is significantly reduced, a high absolute value of theintegral term recorded so far at a high load is directly appliedimmediately after the deceleration, possibly leading to excessivecorrection of the air-fuel ratio.

Also, when the internal combustion engine is operating at a lower loadand lean air-fuel ratio after the engine has been running at a richerair-fuel ratio than the stoichiometric air-fuel ratio for an extendedperiod, the air-fuel ratio will be corrected to be excessively leanimmediately since a correction using the integral term makes theair-fuel ratio even leaner. This may lead to misfire.

The erroneous correction of the air-fuel ratio by an integral term canbe prevented to some extent by setting the integral gain so that theabsolute value of the integral term is relatively small. Setting theintegral gain at a small value, however, may deteriorate air-fuel ratiofeedback convergence, possibly leading to problems, e.g., deterioratedexhaust emissions.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide an air-fuel ratiocontrol apparatus for an internal combustion engine, capable ofadequately preventing erroneous air-fuel ratio correction by an integralterm even if integral correction is adopted for the air-fuel ratio.

In order to achieve the above objective, the present invention providesan air-fuel ratio control apparatus for an internal combustion engine,implementing integral correction of the air-fuel ratio by an integralterm obtained by multiplying an integrated difference between a targetand actual air-fuel ratios by an integral gain, wherein the upper andlower limits of the integral term are set based on an actual intake airamount and air-fuel ratio.

In the present invention, the integral term is limited within a rangebetween the upper and lower limits, which are set based on an actualintake air amount and air-fuel ratio. Therefore, the integral term isprevented from being set at an excessively high or low level which maylead to erroneous air-fuel ratio correction far removed from therealities of the intake air amount and air-fuel ratio.

For example, the upper and lower limits may be set in such a way as toreduce the interval between them, or reduce the absolute value of eachlimit, as the actual intake air amount decreases. This preventsexcessive correction at a low intake air amount while adequately keepingconvergence of the air-fuel ratio feedback control at a high intake airamount, which tends to increase deviation of the air-fuel ratio from itstarget.

Moreover, the upper and lower limits may be set in such a way to limitthe air-fuel ratio correction by the integral term to the lean side asthe actual air-fuel ratio is becoming leaner. This prevents the air-fuelratio from becoming excessively lean as a result of correction by theintegral term.

Limiting the integral term range by setting its upper and lower limitsmay lead to insufficient correction of the air-fuel ratio anddeteriorated convergence of the air-fuel ratio to a target ratio, whenthe actual air-fuel ratio greatly deviates from the target. In such acase, convergence of feedback control of the air-fuel ratio to a targetratio can be ensured by setting the upper and lower limits in such a wayas to allow larger correction of the air-fuel ratio by the integral termto the lean side as an actual air-fuel ratio is continuously leaner thana target ratio, or to allow greater correction of the air-fuel ratio bythe integral term to the rich side as an actual air-fuel ratio iscontinuously richer than a target ratio.

Many internal combustion engines provided with a feedback control systemfor the air-fuel ratio depend on learning control in which a steadystate deviation between actual and target air-fuel ratios, obtainedbased on the history of the differences, is stored as an air-fuel ratiolearning value. Integral correction by an integral term may not simplyconverge to an actual air-fuel ratio at a target ratio, when applied toa learning control system, possibly leading to retarded learning ordeteriorated learning accuracy.

It is preferable in such a case to set the upper and lower limits untila steady state deviation is determined for learning control of theair-fuel ratio in such a way as to have a smaller interval between theupper and lower limits, or smaller absolute value of each limit thanthat after it is determined. Setting the upper and lower limits in thisway can reduce the extent of integral correction of the air-fuel ratiountil the learning of the air-fuel ratio learning value has beencompleted, and keep speed and accuracy of the learning air-fuel ratio atan adequate level while implementing integral correction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatic view showing one embodiment of an air-fuel ratiocontrol apparatus of the present invention;

FIG. 2 is a characteristic curve showing the relationship between theair-fuel ratio and an output voltage from an air-fuel ratio sensor;

FIG. 3 is a characteristic curve showing the relationship between theair-fuel ratio and an output voltage from an oxygen sensor;

FIG. 4 is a flowchart illustrating a procedure for feedback control ofthe air-fuel ratio according to the same embodiment;

FIG. 5 is a flowchart illustrating a procedure for air-fuel ratiolearning control according to the same embodiment;

FIG. 6 is a flowchart illustrating a procedure for correction ratelimiting control according to the same embodiment;

FIG. 7 is a correction rate limiting map according to the sameembodiment;

FIG. 8 is a time chart illustrating the air-fuel ratio feedback controlaccording to the same embodiment;

FIG. 9 is a time chart illustrating the air-fuel ratio feedback controlaccording to the same embodiment; and

FIG. 10 is a time chart illustrating the air-fuel ratio feedback controlwithout correction rate limiting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An air-fuel ratio control apparatus according to one preferredembodiment of the present invention for an internal combustion enginewill now be described by referring to the attached drawings.

FIG. 1 outlines the structure for an internal combustion engine 1 for anautomobile, equipped with an air-fuel ratio control apparatus accordingto the present embodiment of the present invention. The internalcombustion engine 1 is equipped with an intake air passage 2, combustionchambers 3 and an exhaust gas passage 4.

The intake air passage 2 of the internal combustion engine 1 is equippedwith a surge tank 6 and a throttle valve 5 positioned upstream of thetank 6. Opening of the throttle valve 5 varies depending on the extentthat the gas pedal is pressed downward to control the rate of intake airflowing into each combustion chamber 3 via the intake air passage 2(i.e., intake air amount ega).

The intake air passage 2 is equipped with an intake air amount sensor 7,a throttle position sensor 8, and an intake air temperature sensor 9.The intake air amount sensor 7, positioned upstream of the throttlevalve 5, senses intake air amount ega. The throttle position sensor 8 isequipped with an opening sensor which senses opening of the throttlevalve 5 and an idle switch which is on when the throttle valve 5 isfully shut. The intake air temperature sensor 9 senses temperature ofthe intake air (THA) flowing into the internal combustion engine 1.

The intake air passage 2 is also equipped with fuel injection valves 10,which injects fuel supplied under pressure from the fuel tank into theintake air passage 2. The injected fuel is supplied into the combustionchambers 3, after being mixed with air in the intake air passage 2.

The exhaust gas passage 4 in the internal combustion engine 1 isequipped with a three-way catalyst 20, an air-fuel ratio sensor 11, andan oxygen sensor 12. The air-fuel ratio sensor 11 is positioned in theexhaust gas passage 4 upstream of the three-way catalyst 20 and theoxygen sensor 12 is positioned in the exhaust gas passage 4 downstreamof the three-way catalyst 20.

The three-way catalyst 20 exhibits its purification functions forremoving carbon monoxide (CO), hydrocarbons (HCs) and nitrogen oxides(NOx) in exhaust gas most efficiently when the oxygen concentration ofexhaust gas around the catalyst corresponds to an air-fuel ratio nearthe stoichiometric air-fuel ratio. The three-way catalyst 20 in thisembodiment has an oxygen-storage capacity, adsorbing excessive oxygenwhen its concentration of the ambient exhaust gas is excessively high,and releasing oxygen when its concentration is excessively low tocompensate for the shortage. Thus, the three-way catalyst 20autonomously adjusts the ambient oxygen concentration to keep itsexhaust gas purification functions high.

The air-fuel ratio sensor 11 produces voltage which is almost inproportion to the oxygen concentration of the exhaust gas, as shown inFIG. 2. Therefore, the actual air-fuel ratio is detected from the outputvoltage of the air-fuel ratio sensor 11. On the other hand, the outputvoltage of the oxygen sensor 12 greatly depends on whether the air-fuelratio is leaner or richer than the stoichiometric air-fuel ratio. Thus,the output voltage of the oxygen sensor 12 indicates whether the actualair-fuel ratio is richer or leaner than the stoichiometric air-fuelratio.

Each of the combustion chambers 3 in the cylinders of the internalcombustion engine 1 is equipped with an ignition plug 14, to which anignition voltage is applied at a necessary timing by an igniter andignition coil.

The internal combustion engine 1 is cooled by coolant circulatingthrough the cylinder block 1 a. The coolant temperature sensor 17provided at the cylinder block 1 a senses the temperature of thecoolant.

Each of the sensors, i.e., throttle position sensor 8, intake air amountsensor 7, intake air temperature sensor 9, coolant temperature sensor17, air-fuel ratio sensor 11, and oxygen sensor 12, is connected to anelectronic control unit 30 (hereinafter referred to as ECU 30). The ECU30 is composed of a CPU, ROM, RAM, and a microcomputer with a built-inbackup RAM, among others. To the ECU 30, the fuel injection valve 10 andigniter and the like are connected, in addition to the sensors.

The ECU 30 is responsible for controlling various components of theinternal combustion engine 1, e.g., fuel injection valve and igniter.The air-fuel ratio control system in this embodiment is described indetail.

For the three-way catalyst 20 in this embodiment having anoxygen-storage capacity to effectively exhibit its exhaust gaspurification functions, it is necessary to keep a sufficient oxygenstorage capacity while adsorbing a sufficient quantity of oxygen. Thethree-way catalyst 20 can store or release oxygen as required when ithas sufficient capacity (e.g., about half of the maximum capacity storedin the three-way catalyst 20), while adsorbing a sufficient quantity ofoxygen, to always maintain sufficient exhaust gas purificationfunctions.

The ECU 30 in this embodiment implements feedback control for theair-fuel ratio in such a way as to keep the quantity of oxygen stored bythe three-way catalyst 20 at a constant level, in order to allow thecatalyst 20 to stably exhibit the exhaust gas purification functions.More specifically, the ECU 30 senses the difference between a target(i.e., theoretical) and actual air-fuel ratios from output voltage ofthe air-fuel ratio sensor 11, and implements feedback control of theair-fuel ratio by proportional-plus-integral action (PI action) based onthe difference (i.e., PI control).

The PI control of the air-fuel ratio can be implemented by correctingthe air-fuel ratio using an air-fuel ratio correction amount composed ofa proportional term and an integral term, the former being an air-fuelratio difference multiplied by a proportional gain and the latter beingan integrated air-fuel ratio difference multiplied by an integral gain(refer to Formula (1)). It should be noted, however, that the integralterm for the PI control is determined based on the history of air-fuelratios irrespective of the actual intake air amount or the actualair-fuel ratio. This may lead to erroneous air-fuel ratio correctiondepending on the conditions, as described earlier.

The ECU 30 in this embodiment, therefore, implements the PI control ofair-fuel ratio by setting the upper and lower limits of the integralterm based on the actual intake air amount ega and the actual air-fuelratio eabyf to limit the value of the integral term within a rangebetween these limits. This allows the ECU 30 to prevent the integralterm from being set at an excessively high or low level which may leadto erroneous air-fuel ratio correction far removed from the realities ofthe intake air amount ega and the air-fuel ratio eabyf.

Next, the feedback control for the air-fuel ratio in this embodiment isdescribed in detail by referring to the flowchart shown in FIG. 4. TheECU 30 implements the routine shown in FIG. 4 by constant-angleinterruption at every predetermined crank angle.

On starting the interruption processing, the ECU 30 first divides theintake air amount ega sensed by the intake air amount sensor 7 by thestoichiometric air-fuel ratio tabyf (14.6) to obtain a basic injectionamount efcb (Step 102).

Next, the ECU 30 determines whether or not the requirements for feedbackimplementation are satisfied (Step 104). For example, the ECU 30determines that the requirements for feedback implementation aresatisfied when all of the following conditions are met:

-   (1) The coolant temperature is at a predetermined level or higher.-   (2) The internal combustion engine is not being started.-   (3) Fuel supply is not increasing, e.g., for starting the engine.-   (4) Output of the air-fuel ratio sensor 11 has been inverted at    least once.-   (5) Fuel cutoff is not being executed.

The ECU 30, when determining that the feedback implementationrequirements are not satisfied because at least one of the above fiveconditions is not met (Step 104: NO), implements Step 116, and thenimplements Step 114 after setting a feedback correction amount (edfi) at0.

On the other hand, the ECU 30, when determining that the feedbackimplementation requirements are satisfied because all of the above fiveconditions are met (Step 104: YES), implements Step 106, and then Step114 after setting a feedback correction amount (edfii) by the processingin Steps 106 to 112.

In Step 106, the ECU 30 calculates the fuel quantity actually consumedfor combustion (ega/eabyf), based on the actual intake air amount egaand the actual air-fuel ratio eabyf, sensed by the intake air amount andair-fuel ratio sensors 7 and 12, respectively. The ECU 30 calculates afuel difference edfc by subtracting the basic injection amount efcbobtained in Step 102 by the fuel quantity actually consumed forcombustion. The ECU 30 also calculates a new integrated fuel differenceesdfc in Step 106 by adding the fuel difference edfc to the previousintegrated fuel difference esdfc.

In subsequent Step 108, the ECU 30 calculates a proportional term edfipby multiplying the fuel difference edfc by a proportional gain GnFBP.The ECU 30 also calculates a provisional integral term t_edfii bymultiplying the integrated fuel difference esdfc by an integral gainGnFBI.

In subsequent Step 110, the ECU 30 calculates an integral term edfiiafter limiting the value of the provisional integral term t_edfiiobtained in Step 108 with a lower limit correction rate(efafki−t_gddficl) and upper limit correction rate (efafki+t_gddficr).More specifically, the ECU 30 takes an integral term edfii as the lowerlimit correction rate when the provisional integral term t_edfii isbelow the lower limit correction rate, and the integral term edfii asthe upper limit correction rate when the provisional integral termt_edfii is above the upper limit correction rate. Moreover, the ECU 30takes the provisional integral term t_edfii directly as the integralterm edfii when the provisional integral term t_edfii is above the lowerlimit correction rate and, at the same time, below the upper limitcorrection rate. The upper and lower limit correction rates are setbeforehand in the correction amount limiting control, described later.

In subsequent Step 112, the ECU 30 adds the integral term edfii obtainedin Step 110 to the proportional term edfip obtained in Step 108, the sumbeing set as the feedback correction amount edfi.

The ECU 30 sets the feedback correction amount edfi in Step 112 or 116,and adds the feedback correction amount edfi to the basic injectionamount efcb in Step 114 to calculate a final injection amount. Then, theECU 30 multiplies the final injection amount by a coefficient kinj andair-fuel ratio learning value kg to calculate an injector 10energization time etau for fuel injection. The coefficient kinj is thereciprocal of the fuel injection rate (amount of fuel injected per unittime) at the injector 10, and obtained based on fuel pressure or thelike. The air-fuel ratio learning value kg is obtained in the air-fuelratio learning control step, described later.

The air-fuel ratio learning control for calculating the air-fuel ratiolearning value kg is described by referring to the flowchart shown inFIG. 5. The ECU 30 implements the routine shown in FIG. 5 byconstant-angle interruption at every predetermined crank angle. In thisprocessing step, the ECU 30 calculates the air-fuel ratio learning valuekg individually for each of region into which the engine load isdivided.

On starting the processing, the ECU 30 first determines whether or notrequirements for air-fuel ratio learning implementation are satisfied(Step 120). For example, these requirements are satisfied when all ofthe following conditions are met: (1) coolant temperature is at apredetermined level or higher, (2) purging is not being implemented, (3)a load region is within a predetermined range, and (4) fuel cutoff isnot being performed. The ECU 30, when determining that the air-fuelratio learning implementation requirements are satisfied (YES),implements Step 122. When determining that the requirements are notsatisfied (NO), the ECU 30 ends the current processing.

In Step 122, the ECU 30 determines whether or not the actual air-fuelratio eabyf is sufficiently close to a target air-fuel ratio, i.e.,stoichiometric air-fuel ratio (e.g., 14.4≦eabyf<14.8). The ECU 30, whendetermining that the actual air-fuel ratio eabyf converges at a levelclose to the stoichiometric air-fuel ratio (YES), implements Step 124.Otherwise (NO), the ECU 30 ends the current processing.

In Step 124, the ECU 30 determines whether or not the feedback controlis stable, e.g., based on the feedback correction ratio efaf, i.e., theratio of the feedback correction amount (edfi) relative to the basicinjection amount efcb. The ECU 30 determines that the air-fuel feedbackcontrol is stable, when the absolute value of the feedback correctionratio efaf is below 2%, and it is unstable when the absolute value ofthe feedback correction ratio efaf is 2% or more. The ECU 30, whendetermining that the air-fuel feedback control is stable (YES),implements Step 126. Otherwise (NO), the ECU 30 implements Step 130.

When the processing step proceeds to Step 130, the ECU 30 renews theair-fuel ratio learning value kg in the load region in such a way as toreduce the absolute value of the feedback correction ratio efaf, andthen ends the current processing.

When the processing step proceeds to Step 126, on the other hand, theECU 30 determines whether or not the air-fuel ratio feedback control hasbeen stably working continuously for more than a predetermined time. TheECU 30, when determining that the air-fuel feedback control has beenstable continuously for the predetermined time (YES), implements Step128. Otherwise (NO), the ECU 30 ends the current processing.

In Step 128, the ECU 30 determines that the air-fuel ratio learning inthe load region is temporarily completed and ends the current processingafter storing the air-fuel ratio learning value kg and the history ofcompletion of the learning in a backup RAM. The history is kept untildata stored in the backup RAM are erased, e.g., by replacing the batterywith a new one.

The correction amount limiting control is described by referring to theflowchart shown in FIG. 6. This step calculates the lower and upperlimit correction rates, which limit the value of an integral term edfiifor the air-fuel ratio feedback control. The ECU 30 implements theroutine shown in FIG. 5 by constant-angle interruption at everypredetermined crank angle.

When this processing starts, the ECU 30 first determines whether or notrequirements for feedback implementation are satisfied (Step 140). Thisdetermination is implemented in a manner similar to that for theair-fuel ratio feedback control in Step 104, illustrated in FIG. 4. TheECU 30, when determining that the implementation requirements aresatisfied (YES), implements Step 142. Otherwise (NO), it implements Step156, where it sets a basic correction rate efafki at 0 beforetemporarily stopping the processing routine.

In Step 142, on the other hand, the ECU 30 determines whether an actualair-fuel ratio eabyf is at the stoichiometric level or richer or leanerthan this level. The ECU 30, when determining that the air-fuel ratioeabyf is richer than the stoichiometric level, implements Step 144 tosubtract from the basic correction rate efafki a correction ratedifference Δki, and then implements Step 148. When determining that theair-fuel ratio eabyf is leaner than the stoichiometric level, on theother hand, the ECU 30 implements Step 146 to add a correction ratedifference Δki to the basic correction rate efafki, and then implementsStep 148. When determining that the air-fuel feedback control is at thestoichiometric level, the ECU 30 implements Step 148 directly with thebasic correction rate efafki as it is.

The value for the correction rate difference Δki is set according to themagnitude of the intake air amount ega. More specifically, it is set insuch a way as to increase as the intake air amount ega increases.Therefore, the larger the intake air amount ega is, the greater thebasic correction rate is changed.

The basic correction rate efafki is a fuel injection correction rateserving as a standard, based on which of the upper and lower limits ofan integral term edfii are set. The basic correction rate efafki isdetermined based on the history of air-fuel ratios, as describedearlier. More specifically, the basic correction rate efafki isgradually varied to reduce the fuel injection amount when the actualair-fuel ratio eabyf is continuously richer than the stoichiometricair-fuel ratio, and to increase the fuel injection amount when the ratioeabyf is continuously leaner than the stoichiometric air-fuel ratio, inorder to correct the fuel injection amount.

In Step 148, the ECU 30 calculates a decrease limiting value t_gddficland an increase limiting value t_gddficr in accordance with themagnitudes of the actual air-fuel ratio eabyf and intake air amount egaby referring to the map given in FIG. 7. As illustrated in FIG. 7, botha decrease in the limiting value t_gddficl and an increase in thelimiting value t_gddficr are set in such a way as to converge to 0 asthe intake air amount decreases.

In Step 150, the ECU 30 determines whether or not there is an air-fuelratio learning history in an actual load region. When determining thatthere is no air-fuel ratio learning history (NO), the ECU 30 implementsStep 152 and then Step 154. Otherwise (YES), the ECU 30 directlyimplements S154 bypassing Step 152.

In Step 152, the ECU 30 changes the decrease limiting value t_gddficl orthe increase limiting value t_gddficr to a level close to 0,irrespective of the intake air amount or the air-fuel ratio (asindicated by the broken line shown in FIG. 7).

In Step 154, the ECU 30 sets upper and lower limits (upper and lowercorrection rates, respectively) for a reduced correction rate for theintegral term edfii, the former being the basic correction rate efafkiadded with the increase limiting value t_gddficr, and the latter beingthe basic correction rate efafki subtracted by the decrease limitingvalue t_gddficl. The reduced correction rate for the integral term edfiimeans the integral term efafki divided by the basic injection amountefcb. Then, the ECU 30 temporarily stops the processing routine.

The correction amount limiting control described above sets upper andlower limits for the integral term edfii (more strictly, a reducedcorrection rate for the integral term edfii) based on the basiccorrection rate efafki and the increase limiting value t_gddficr or thedecrease limiting value t_gddficl. The increase limiting value t_gddficrand decrease limiting value t_gddficl are set by the actual intake airamount ega and the actual air-fuel ratio eabyf, respectively. In thisembodiment, therefore, the integral term edfii is limited within theupper and lower limits determined in accordance with the actual intakeair amount ega and the actual air-fuel ratio eabyf. This limitationprevents an integral term from being set at an excessively high or lowlevel which may lead to erroneous air-fuel ratio correction removed fromthe realities of the intake air amount ega and the actual air-fuel ratioeabyf.

More specifically, the increase limiting value t_gddficr and thedecrease limiting value t_gddficl are set in such a way as to reduce theinterval between the upper and lower limits of the integral term edfii,or reduce the absolute value of each limit, as the actual intake airamount ega decreases. This prevents excessive correction at a low intakeair amount while adequately maintaining convergence for the air-fuelratio feedback control at a high intake air amount, which tends toincrease deviation of the air-fuel ratio from its target.

Moreover, the increase limiting value t_gddficr and the decreaselimiting value t_gddficl are set in such a way as to limit the actualair-fuel ratio eabyf when it is lean, i.e., in such a way as to limitthe air-fuel ratio correction by the integral term edfii to the leanside. This prevents the air-fuel ratio from becoming excessively lean asa result of correction by the integral term edfii.

It should be noted that simply limiting the range for the integral termedfii by setting the upper and lower limits may deteriorate convergenceof the air-fuel ratio eabyf to a target ratio when there is asignificant difference between them, because of insufficient air-fuelratio correction. In this regard, this embodiment varies the basiccorrection rate efafki in accordance with the actual air-fuel ratioeabyf, as illustrated in FIG. 8. More specifically, it sets the upperand lower limits of the integral term edfii in such a way as to allowlarger correction of the air-fuel ratio to the lean side as the actualair-fuel ratio eabyf is continuously leaner than the target ratio, or toallow larger correction of the air-fuel ratio by the integral term tothe rich side as the actual air-fuel ratio is continuously richer thanthe target ratio. This secures convergence of the air-fuel ratiofeedback control to the target ratio.

On the other hand, this embodiment implements the air-fuel ratiolearning control with a stored air-fuel ratio learning value kg, whichis a steady state deviation between the actual air-fuel ratio eabyf andthe stoichiometric air-fuel ratio, obtained from the history of thedifferences in the air-fuel ratio feedback control. The air-fuel ratiomay not be simply converged to the target air-fuel ratio depending onthe transition of the actual air-fuel ratio eabyf to that point. Thispossibly leads to retarded learning or deteriorated learning accuracy.

In this regard, this embodiment sets the increase limiting valuet_gddficr and the decrease limiting value t_gddficl in such a way as toreduce the interval between the upper and lower limits of the integralterm edfii, or reduce the absolute value of each limit, until completionof a steady state deviation calculation, i.e., the air-fuel ratiolearning value kg calculation, in the air-fuel ratio learning control.This controls integral correction of the air-fuel ratio to a relativelylimited extent before completion of learning with the air-fuel ratiolearning value kg to suitably maintain learning speed and accuracy.

FIG. 9 shows the actual air-fuel ratio eabyf and the feedback correctionratio efaf (i.e., the feedback correction amount edfi divided by thebasic injection amount efcb) changing with time. As described above,this embodiment sets the upper and lower limits in such a way as to keepthe integral term edfii at a value close to 0 until completion of theair-fuel ratio learning value kg calculation, as a result of which theair-fuel ratio feedback control is mainly implemented by theproportional correction, with essentially no integral correction.Therefore, the actual air-fuel ratio eabyf promptly converges to a valueclose to the stoichiometric air-fuel ratio, as illustrated in FIG. 9, toalso promptly complete the air-fuel ratio learning control.

On the other hand, FIG. 10 shows the actual air-fuel ratio eabyf and thefeedback correction ratio efaf changing with time similar to thelearning control as with the air-fuel ratio learning value kg butwithout limiting the range of the integral term edfii by the upper andlower limits. Integral correction of the air-fuel ratio without limitingrange of the integral term causes retarded learning and deterioratedlearning accuracy, resulting from deteriorated convergence andinstability of the actual air-fuel ratio eabyf, as illustrated in FIG.10.

The following modifications of the above embodiment are also within thescope of the present invention.

Step 152 described above sets the increase limiting value t_gddficr andthe decrease limiting value t_gddficl in the case of no history forair-fuel ratio learning. In one modification, these values may be set at0 (cleared). This fixes the integral term edfii at the basic correctionrate efafki to further limit integral correction of the air-fuel ratio,thereby further reducing retarded learning or deteriorated learningaccuracy with the air-fuel ratio learning value kg, which may resultfrom integral correction.

Steps 150 and 152 in the above embodiment, which implement correctionamount limiting control, may be skipped, when an adverse effect ofintegral correction of air-fuel ratio on learning time or accuracy isnegligible, e.g., when no air-fuel ratio learning control isimplemented.

The above embodiment varies the basic correction rate efafki inaccordance with the history of the actual air-fuel ratios eabyf.However, the basic correction rate efafki may be set at a fixed value,e.g., 0. The range of the integral term edfii is limited also in thiscase in accordance with the actual intake air amount ega and the actualair-fuel ratio eabyf. Therefore, this modification prevents erroneousair-fuel correction, removed from the realities of actual intake airamount ega and the actual air-fuel ratio eabyf.

Step 112 for finding the feedback correction amount edfi, illustrated inFIG. 4, may be modified to add a derivative action, i.e., to implementproportional plus integral plus derivative (PID) action, where aderivative term, which is a product of a derivative of fuel differenceand a derivative gain, is additionally included in the feedbackcorrection amount edfi.

The present invention is applicable to various internal combustionengines, not limited to the port injection type illustrated in FIG. 1,with fuel injected into an air intake port. For example, it isapplicable to a cylinder injection type with fuel directly injected intoa cylinder.

1. An air-fuel ratio control apparatus for an internal combustionengine, the apparatus comprising a control unit that includes controllogic implementing integral correction of the air-fuel ratio with anintegral term, the integral term being obtained by multiplying anintegrated difference between a target air-fuel ratio and the actualair-fuel ratio by an integral gain, wherein an upper limit value and alower limit value of the integral term are set based on an actual intakeair amount and an actual air-fuel ratio.
 2. The air-fuel ratio controlapparatus for an internal combustion engine according to claim 1,wherein the upper and lower limit values are set in such a way to reducethe interval between the limit values as the actual intake air amountdecreases.
 3. The air-fuel ratio control apparatus for an internalcombustion engine according to claim 1, wherein the upper and lowerlimit values are set in such a way to reduce the absolute value of eachlimit value as the actual intake air amount decreases.
 4. The air-fuelratio control apparatus for an internal combustion engine according toclaim 1, wherein the upper and lower limit values are set in such a waythat air-fuel ratio correction with the integral term for a leanair-fuel ratio is limited as the actual air-fuel ratio becomes leaner.5. The air-fuel ratio control apparatus for an internal combustionengine according to claim 1, wherein the upper and lower limit valuesare set in such a way to allow larger correction of the air-fuel ratiowith the integral term for a lean air-fuel ratio as the actual air-fuelratio remains leaner than the target ratio for a longer period.
 6. Theair-fuel ratio control apparatus for an internal combustion engineaccording to claim 1, wherein the upper and lower limit values are setin such a way to allow larger correction of air-fuel ratio with theintegral term for a rich air-fuel ratio as the actual air-fuel ratioremains richer than the target ratio for a longer period.
 7. Theair-fuel ratio control apparatus for an internal combustion engineaccording to claim 1, wherein air-fuel ratio learning control isimplemented, in which a steady state deviation between the actualair-fuel ratio and the target air-fuel ratio is computed based on thehistory of difference between the air-fuel ratios, and the computedsteady state deviation is stored as a learning value, and wherein, untilthe computation of the steady state deviation is completed, the upperand lower limit values are set in such a way to have a smaller intervalbetween the limit values than that after the computation of the steadystate deviation is completed.
 8. The air-fuel ratio control apparatusfor an internal combustion engine according to claim 1, wherein air-fuelratio learning control is implemented, in which a steady state deviationbetween the actual air-fuel ratio and the target air-fuel ratio iscomputed based on a history of difference between the air-fuel ratios,and the computed steady state deviation is stored as a learning value,and wherein, until the computation of the steady state deviation iscompleted, the upper and lower limits are set in such a way to each havea smaller absolute value than that after the computation of the steadystate deviation is completed.
 9. The air-fuel ratio control apparatusfor an internal combustion engine according to claim 2, wherein theupper and lower limit values are set in such a way to allow largercorrection of the air-fuel ratio with the integral term for a leanair-fuel ratio as the actual air-fuel ratio remains leaner than thetarget ratio for a longer period.
 10. The air-fuel ratio controlapparatus for an internal combustion engine according to claim 3,wherein the upper and lower limit values are set in such a way to allowlarger correction of the air-fuel ratio with the integral term for alean air-fuel ratio as the actual air-fuel ratio remains leaner than thetarget ratio for a longer period.
 11. The air-fuel ratio controlapparatus for an internal combustion engine according to claim 4,wherein the upper and lower limit values are set in such a way to allowlarger correction of the air-fuel ratio with the integral term for alean air-fuel ratio as the actual air-fuel ratio remains leaner than thetarget ratio for a longer period.
 12. The air-fuel ratio controlapparatus for an internal combustion engine according to claim 2,wherein the upper and lower limit values are set in such a way to allowlarger correction of air-fuel ratio with the integral term for a richair-fuel ratio as the actual air-fuel ratio remains richer than thetarget ratio for a longer period.
 13. The air-fuel ratio controlapparatus for an internal combustion engine according to claim 3,wherein the upper and lower limit values are set in such a way to allowlarger correction of air-fuel ratio with the integral term for a richair-fuel ratio as the actual air-fuel ratio remains richer than thetarget ratio for a longer period.
 14. The air-fuel ratio controlapparatus for an internal combustion engine according to claim 4,wherein the upper and lower limit values are set in such a way to allowlarger correction of air-fuel ratio with the integral term for a richair-fuel ratio as the actual air-fuel ratio remains richer than thetarget ratio for a longer period.